It is known that by visbreaking is meant a treatment of heavy hydrocarbon feedstocks which comprises placing said feedstocks in the liquid state into a furnace at a temperature sufficient to cause the heaviest hydrocarbons to crack and then introducing them into a maturation device (known in the art as a "soaker") wherein, without additional heating, they travel at a rate such that at the prevailing temperature they have a sufficient residence time for achieving the desired cracking of the heavy molecules into lighter molecules. The cracking results in a reduction in viscosity of the treated feedstock. This process is known as visbreaking (a term of art used as an abbreviation for "viscosity reduction"), and the apparatus used is known as a visbreaker.
The soaker usually has the form of a cylindrical enclosure which is not provided with additional means for heating the feedstock and in which, because cracking is endothermic, the feedstock temperature drops a few tens of degrees between the time the feedstock enters the soaker and the time it exits. The temperature in the soaker is generally about 400-500.degree. C. an the pressure about 2 to 30.times.10.sup.5 pascal. The residence time of the feedstock in the soaker is about 10-30 minutes. The severity, which is a function of the residence time and the soaker temperature, is of the order of 20 minutes.
The feedstock to be treated is injected at the bottom of the soaker, whereas the cracked product, including any gaseous products that may have formed, is discharged at the top and is directed to a fractionation unit for atmospheric distillation followed by vacuum distillation.
The feedstock to be treated can be a heavy petroleum crude, an atmospheric distillation residue, used only rarely because there are other ways of utilizing it, a vacuum distillation residue or a deasphalting pitch.
After fractionation, the visbroken products consist of gaseous hydrocarbons, liquefied petroleum gas, gasoline, gas oil, distillate and visbroken vacuum residue.
The visbroken vacuum residue is the last recoverable product and, to be used as fuel oil base, must meet stringent requirements of stability and compatibility with other petroleum fractions. Hence, to meet these requirements, the operator must adjust the visbreaking conditions, particularly the temperature.
A major problem encountered in visbreaking units lies in the nonuniform travel of the charge stock inside the soaker and in back-mixing and vortexing, occurring particularly in the vicinity of the side walls and at the bottom of the soaker. These disturbances are aggravated by the gases generated by the cracking reactions and by the fact that the residence time of the feedstock in the soaker varies markedly in the same crosssection, depending on the zone considered. As a result, there is a risk that part of the treated feedstock will be overcracked, while another fraction will be insufficiently cracked.
To eliminate this drawback, it has been proposed in EP-A-007 656 to dispose inside the soaker, transversely to the direction of feedstock flow, a plurality of internal structures consisting of perforated plates, the orifices in said plates being circular and/or having the shape of slits, said orifices preferably constituting from 1 to 30% of the plate surface area.
Thus, at the level of the plate orifices which are traversed by the gas bubbles present, each plate causes mixing of the feedstock. The aforecited European Patent Application recommends that from 1 to 20 plates of this type be used in a soaker.
As indicated in EP-A-0 138 247 (and in equivalent U.S. Pat. No. 4,551,233 issued Dec. 5, 1985), when plates of this type are used, however, the stability of the cracked products is insufficient, particularly when large quantities of gaseous compounds are formed and considerable amounts of coke appear, which entails a serious risk of plugging the plate perforations. This results in extended, costly soaker shut-downs to clean the perforated plates and remove the coke present.
Document FR-A-2 528 444 (see equivalent GB-A-2,133,034) proposes a process for thermal cracking of hydrocarbon oils whereby a fluid, such as steam, is introduced tangentially into the lower part of the soaker (see page 6, lines 8 to 17) through nozzles. The purpose of such introduction is to impart rotation to the hydrocarbon feedstock.
Imparting rotation to the feedstock, however, requires very large amounts of steam which implies a limitation of the space available for the feedstock in the soaker and, hence, a reduction in its residence time, which is prejudicial to visbreaking.